The present invention relates generally to the field of protective circuitry for electrical loads, such as electric motors. More particularly, the invention relates to a device for providing power to a load, as well as for providing protection from overloads and instantaneous trips.
Many applications exist for electrical loads such as electric motors. Motors are pervasive throughout all industry, commercial applications, residential applications, and so forth. In a typical motor installation, thermal overload protection is provided for interrupting electrical power to the motor under certain conditions. For example, depending upon the motor rating, the overload protection will cause an interruption or trip to occur based upon relatively slow heating conditions. While overcurrent conditions are common during startup of a motor, such conditions during normal operation should not generally exist over extended periods. If these conditions do persist, the overload protection prevents damage to the motor or wiring by tripping the circuit.
Another type of protection that is commonly provided for electrical loads such as motors is instantaneous overload protection. A common device used for instantaneous tripping, in general, the instantaneous trip device operates much more quickly than the thermal overload protective devices, thereby providing multiple types and levels of overload protection.
In current designs, different specifically-designed devices and combination of devices are provided for the desired thermal and instantaneous circuit interruption for protecting electrical loads. The resulting product offerings, then, typically include a large number of different components arranged in different combinations which must be selected, installed, serviced, and monitored in an installation, depending upon the variety of motors and motor sizes present. A difficulty in avoiding this consequence resides in the design of the protected devices and their combination. In particular, conventional designs call for instantaneous trip devices that are sized to interrupt current at a multiple of the full load amperage (FLC), typically on the order of 13 times the FLC. At the same time, a typical thermal overload protective circuit may be designed to trip more slowly, but at lower current levels, such as at 7–8 times FLC. While the difference in these multiples provides a “hole” in the levels of designed trip current, this hole is typically accepted in the art.
There is a need, therefore, for improved designs of circuit protective devices, such as devices used with electric motors. There is a particular need for a design which can provide an extended trip range, thereby reducing the number of separate components and component combinations required to provide adequate circuit interruption capability for a wide range of motors, motor ratings, and frame sizes.